Various techniques of sheet processing using a processing section including a pair of corrugated rollers have been proposed, in which a sheet such as nonwoven fabric is introduced into a bite between a counterrotating pair of corrugated rollers each having ridges extending on the peripheral surface parallel to the axis of rotation in intermeshing engagement with each other.
For example, patent literature 1 below proposes a method for stretching a sheet such as nonwoven fabric by adjusting the depth of engagement of the roller ridges.
When a sheet is subjected to stretch processing by the passage between a pair of corrugated rollers as in the method disclosed in patent literature 1, although a high stretch ratio can be reached by increasing the depth of engagement, an increase in depth of engagement tends to result in reduction of sheet strength.
Patent literature 2 below proposes a technique for performing high stretch ratio processing in which the depth of engagement of gear rollers is adjusted and, in addition, the sheet such as nonwoven fabric is preliminarily stretched by stretching rollers prior to the stretching between the gear rollers.
According to the technique of patent literature 2, however, because the rotational velocity of the pair of gear rollers is set higher than that of the upstream stretching rollers, a tension in the machine direction is added to the sheet such as nonwoven fabric so that the sheet reduces in width before being stretched between the gear rollers. Also, because of the high rotational velocity of the pair of gear rollers, the depth of grooves between ridges cannot be set large enough to provide the resulting stretchable sheet with improved feel to the touch.    Patent literature 1: JP 2003-73967A    Patent literature 2: US 2010065984A1